Apparatus for controlling the air-fuel ratio of an internal combustion engine

Abstract

The invention is directed to an apparatus for controlling the air-fuel ratio, in particular of an internal combustion engine. The apparatus includes a sensor responsive to this ratio such as an oxygen sensor or the like and a circuit arrangement for evaluating the sensor output signal. The circuit arrangement includes means to compensate for the offset voltage influence thereby permitting a precise control of the air-fuel ratio in a Lambda range in which the dependent relationship between the sensor output and the sensor input signals is barely distinctive. For this purpose, the influence of the offset voltage of the control amplifiers used is measured at regular or irregular intervals and stored in analog or digital format. On the one hand, this stored quantity may be used to apply to the inputs of the control amplifiers a compensating voltage acting in opposition to the input offset voltage or, on the other hand, to subtract from the operational amplifier output the voltage portion resulting from offset voltages. The apparatus of the invention is also suitable for controlling the air-fuel ratio of heating equipment.

What is claimed is: 
     
       1. Apparatus for controlling the air-fuel ratio of an internal combustion engine comprising: a lambda probe responsive to changes in said ratio and monitoring said ratio over a range of lambda values that includes a region wherein the output voltages of the probe exhibit only a weak dependence upon inputs to the probe; and,   a circuit arrangement for evaluating the output of said probe and including: an operational amplifier wherein an offset voltage occurs because of drift in said circuit arrangement and temperature variations to which said probe is subjected during operation thereof; and, compensation means for compensating for said offset voltage thereby permitting a precise regulation of the air-fuel ratio in said region, said compensation means including circuit means for compensating for said offset voltage of said operational amplifier, said circuit means including switching means for switching said operational amplifier from the operation mode into and out of a compensation mode in a sequence corresponding to segments of time when it is not necessary or possible to conduct a determination of the lambda value.   
     
     
       2. The apparatus of claim 1, said switching means including means for setting the input voltage of said operational amplifier to zero whereby the output voltage of said amplifier is said offset voltage and for switching said operational amplifier back into said operation mode; and, said circuit means including: storage means for receiving and storing said offset voltage during said compensation mode and for subtracting said stored offset voltage from the voltage at the output of said operational amplifier after said switching means switches said amplifier back into said operation mode.   
     
     
       3. The apparatus of claim 2, said storage means including means for storing said processed voltage for an inlimited time; and, said circuit means including means for switching said operational amplifier into and out of said compensation mode at a preselected repetition frequency. 
     
     
       4. The apparatus of claim 2 said storage means including means for storing said processed voltage for an unlimited time; and, said circuit means including means for switching said operational amplifier into and out of said compensation mode in a sequence to correspond to segments of time when it is not necessary or possible to conduct a determination of the lambda value. 
     
     
       5. The apparatus of claim 2, said means for storing said processed voltage being analog means. 
     
     
       6. The apparatus of claim 2, said processing circuit means comprising: an analog-to-digital convertor for converting said output voltage into a digital quantity; a microcomputer for processing said digital quantity, said storage means being part of said microcomputer for storing said digital quantity; and, a digital-to-analog convertor for converting said digital quantity back to an analog quantity. 
     
     
       7. The apparatus of claim 2, said operational amplifier being a subtracting operational amplifier connected to said lambda probe for receiving the output thereof. 
     
     
       8. The apparatus of claim 2, said processing circuit means being configured so as to permit both lean and rich air-fuel ratios to be controlled in the region of λ˜1.2 and of λ˜0.8, respectively, with a relative accuracy lying in the range of one part per mil. 
     
     
       9. The apparatus of claim 8, said processing circuit means being further configured to provide a continuous control of the air-fuel ratio. 
     
     
       10. Apparatus for controlling the air-fuel ratio of an internal combustion engine comprising: a lambda probe responsive to changes in said ratio and monitoring said ratio over range of lambda values that includes a region wherein the output voltages of the probe exhibit only a weak dependence upon inputs to the probe; and,   a circuit arrangement for evaluating the output of said probe and including an operational amplifier wherein an offset voltage occurs because of drift in said circuit arrangement and temperature variations to which said probe is subjected during operation thereof; and, compensation means for compensating for said offset voltage thereby permitting a precise regulation of the air-fuel ratio in said region, said compensation means including circuit means for compensating for the influence of said offset voltage of said operational amplifier;   said circuit means including:   switching means for switching said operational amplifier from the operation mode into a compensation mode by setting the input voltage of said amplifier to zero whereby the output voltage of said operational amplifier is caused exclusively by said offset voltage and for switching said operational amplifier back into said operation mode;   processing circuit means for processing said output voltage and applying the processed output voltage to the input of said operational amplifier whereby the output of said amplifier is reduced to zero; and,   storage means for storing said processed output voltage until the operational amplifier is again switched into the compensation mode.   
     
     
       11. The apparatus of claim 10, said storage means including means for storing said processed voltage for an unlimited time; and, said circuit means including means for switching said operational amplifier into and out of said compensation mode at a preselected repetition frequency. 
     
     
       12. The apparatus of claim 10, said storage means including means for storing said processed voltage for an unlimited time; and, said circuit means including means for switching said operational amplifier into and out of said compensation mode in a sequence to correspond to segments of time when it is not necessary or possible to conduct a determination of the lambda value. 
     
     
       13. The apparatus of claim 10, said means for storing said processed voltage being analog means. 
     
     
       14. The apparatus of claim 10, said processing circuit means comprising: an analog-to-digital convertor for converting said output voltage into a digital quantity; a microcomputer for processing said digital quantity, said storage means being part of said microcomputer for storing said digital quantity; and, a digital-to-analog convertor for converting said digital quantity back to an analog quantity. 
     
     
       15. The apparatus of claim 10, said operational amplifier being a subtracting operational amplifier connected to said lambda probe for receiving the output thereof. 
     
     
       16. The apparatus of claim 10, said processing circuit means being configured so as to permit both lean and rich air-fuel ratios to be controlled in the region of λ˜1.2 and of λ˜0.8, respectively, with a relative accuracy lying in the range of one part per mil. 
     
     
       17. The apparatus of claim 16, said processing circuit means being further configured to provide a continuous control of the air-fuel ratio. 
     
     
       18. Apparatus for controlling the air-fuel ratio of heating equipment comprising: a lambda probe responsive to changes in said ratio and monitoring said ratio over a range of lambda values that includes a region wherein the output voltages of the probe exhibit only a weak dependence upon inputs to the probe; and,   a circuit arrangement for evaluating the output of said probe and including an operational amplifier wherein an offset voltage occurs because of drift in said circuit arrangement and temperature variations to which said probe is subjected during operation thereof; and, compensation means for compensating for said offset voltage thereby permitting a precise regulation of the air-fuel ratio in said region, said compensation means including circuit means for compensating for said offset voltage of said operational amplifier, said circuit means including switching means for switching said operational amplifier from the operation mode into and out of a compensation mode in a sequence corresponding to segments of time when it is not necessary or possible to conduct a determination of the lambda value.   
     
     
       19. The apparatus of claim 18, said switching means including means for setting the input voltage of said operational amplifier to zero whereby the output voltage of said amplifier is said offset voltage and for switching said operational amplifier back into said operation mode; and, said circuit means including: storage means for receiving and storing said offset voltage during said compensation mode and for subtracting said stored offset voltage from the voltage at the output of said operational amplifier after said switching means switches said amplifier back into said operation mode.   
     
     
       20. Apparatus for controlling the air-fuel ratio of heating equipment comprising: a lambda probe responsive to changes in said ratio and monitoring said ratio over a range of lambda values that includes a region wherein the output voltages of the probe exhibit only a weak dependence upon inputs to the probe; and,   a circuit arrangement for evaluating the output of said probe and including an operational amplifier wherein an offset voltage occurs because of drift in said circuit arrangement and temperature variations to which said probe is subjected during operation thereof; and, compensation means for compensating for said offset voltage thereby permitting a precise regulation of the air-fuel ratio in said region, said compensation means including circuit means for compensating for the influence of said offset voltage of said operational amplifier;   said circuit means including:   switching means for switching said operational amplifier from the operation mode into a compensation mode by setting the input voltage of said amplifier to zero whereby the output voltage of said operational amplifier is caused exclusively by said offset voltage and for switching said operational amplifier back into said operational mode;   processing circuit means for processing said output voltage and applying the processed output voltage to the input of said operational amplifier whereby the output of said amplifier is reduced to zero; and,   storage means for storing said processed output voltage until the operational amplifier is again switched into the compensation mode.   
     
     
       21. The apparatus of claim 20 comprising means for controlling an extremely lean air-fuel ratio of up to lambda having a value of 1.8. 
     
     
       22. The apparatus of claim 19 comprising means for controlling an extremely lean air-fuel ratio of up to lambda having a value of 1.8.